A wide variety of cardiac pacemakers are known and commercially available. Pacemakers are generally characterized by which chambers of the heart they are capable of sensing, the chambers to which they deliver pacing stimuli, and their responses, if any, to sensed intrinsic electrical cardiac activity. Some pacemakers delivery pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly, however, pacemakers sense electrical cardiac activity in one or both of the chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based on the occurrence and recognition of sensed intrinsic electrical events. A so-called "VVI" pacemaker, for example, senses electrical cardiac activity in the ventricle of the patient's heart, and delivers pacing stimuli to the ventricle only in the absence of electrical signals indicative of natural ventricular contractions. A "DDD" pacemaker, on the other hand, senses electrical signals in both the atrium and ventricle of the patient's heart, and delivers atrial pacing stimuli in the absence of signals indicative of natural atrial contractions, and ventricular pacing stimuli in the absence of signals indicative of natural ventricular contractions. The delivery of each pacing stimulus by a DDD pacemaker is synchronized with prior sensed or paced events.
Pacemakers are also known which respond to other types of physiologically-based signals, such as signals from sensors for measuring the pressure inside the patient's ventricle or for measuring the level of the patient's physical activity. In recent years, pacemakers which measure the metabolic demand for oxygen and vary the pacing rate in response thereto have become widely available. Perhaps the most popularly employed method for measuring the need for oxygenated blood is to measure the physical activity of the patient by means of a piezoelectric transducer. Such a pacemaker is disclosed in U.S. Pat. No. 4,485,813 issued to Anderson et al.
In typical prior art rate-responsive pacemakers, the pacing rate is determined according to the output from an activity sensor. The pacing rate is variable between a predetermined maximum and minimum level, which may be selectable by a physical from among a plurality of programmable upper and lower rate limit settings. When the activity sensor output indicates that the patient's activity level has increased, the pacing rate is increased from the programmed lower rate by an incremental amount which is determined as a function of the output of the activity sensor. That is, the rate-responsive or "target" pacing rate in a rate-responsive pacemaker is determined as follows: EQU Target Rate=Programmed Lower Rate+f(sensor output)
where f is typically a linear or monotonic function of the sensor output. As long as patient activity continues to be indicated, the pacing rate is periodically increased by incremental amounts calculated according to the above formula, until the programmed upper rate limit is reached. When patient activity ceases, the pacing rate is gradually reduced, until the programmed lower rate limit is reached.
In an effort to minimize patient problems and to prolong or extend the useful life of an implanted pacemaker, it has become common practice to provide numerous programmable parameters in order to permit the physical to select and/or periodically adjust the desired parameters or to match or optimize the pacing system to the patient's physiologic requirements. The physician may adjust the output energy settings to maximize pacemaker battery longevity while ensuring an adequate patient safety margin. Additionally, the physician may adjust the sensing threshold to ensure adequate sensing of intrinsic depolarization of cardiac tissue, while preventing oversensing of unwanted events such as myopotential interference or electromagnetic interference (EMI). Also, programmable parameters are typically required to enable and to optimize a pacemaker rate response function. For example, Medtronic, Inc.'s Legend and Activitrax series of pacemakers are multiprogrammable, rate-responsive pacemakers having the following programmable parameters: pacing mode, sensitivity, refractory period, pulse amplitude, pulse width, lower and upper rate limits, rate response gain, and activity threshold.
For any of the known rate-responsive pacemakers, it is clearly desirable that the sensor output correlate to as high a degree as possible with the actual metabolic and physiologic needs of the patient, so that the resulting rate-responsive pacing rate may be adjusted to appropriate levels. A piezoelectric activity sensor can only be used to indirectly determine the metabolic need. The physical activity sensed can be influenced by upper body motion. Therefore, an exercise that involves arm motion may provide signals that are inappropriately greater than the metabolic need. Conversely, exercises that stimulate the lower body only, such as bicycle riding, may provide a low indication of metabolic need while the actual requirement is very high. Therefore, it would be desirable to implement a rate-responsive pacemaker that is based on a parameter that is correlated directly to metabolic need.
Minute ventilation (V.sub.6) has been demonstrated clinically to be a parameter that correlates directly to the actual metabolic and physiologic needs of the patient. Minute ventilation is defined by the equation: EQU V.sub.c =RR.times.VT
where RR=respiration rate in breaths per minute (bpm), and VT=tidal volume in liters. Clinically, the measurement of V.sub.c is performed by having the patient breathe directly into a device that measurements the exchange of air and computes the total volume per minute. The direct measurement of V.sub.c is not possible with an implanted device. However, measurement of the impedance changes of the thoracic cavity can be implemented with an implanted pacemaker. Such a pacemaker is disclosed in U.S. Pat. No. 4,702,253 issued to Nappholz et al. on Oct. 27, 1987. The magnitude of the change of the impedance signal corresponds to the tidal volume and the frequency of change corresponds to respiration rate.
The use of transthoracic impedance to indicate V.sub.c has a significant spurious false positive due to upper body myopotential interference and postural changes. Further, slow-acting physiologic parameters such as transitory blood chemistry changes also impact the impedance amplitude. Therefore, it may be desirable to define a rate response function f which minimizes the effects of spurious or transitory changes in impedance sensor output which do not accurately indicate the patient's metabolic needs.
Additionally, basing the pacing rate solely on V.sub.c does not provide the optimum pacing rate increase at the onset of exercise. The VT and RR have an inherent physiologic time delay due to the response of the CO.sub.2 receptors and the autonomic nervous system. The increase in V.sub.c lags behind the need for the increased cardiac output. Therefore, it may also be desirable to implement a rate response function f that is based on a combination of a fast responding sensor such as an activity sensor and a physiologically delayed metabolic sensor such as V.sub.c.
The combination of the activity and V.sub.c sensor outputs for a rate response function in a manner where the faster of the two independently derived target pacing rates would be utilized as the actual pacing rate is believed to be effective. Such an `OR` combination of sensor signals is disclosed, for example, in U.S. Pat. No. 5,063,927 issued to Webb et al., which patent is incorporated herein by reference. Combining an activity-based target rate and a metabolic-based target rate in the manner suggested by Webb et al. would provide the fast onset of an activity sensor with the sustained response of a V.sub.c sensor. Provisions in the rate response function f would need to include lower and upper rate limits, along with a mapping function from impedance to pacing rate that could be adjusted by a physician to optimize the function for each patient.